Air Conditioning and Heating Sizing | HVAC Technical

Air Conditioning and Heating Sizing - It is not uncommon for HVAC estimators or HVAC contractors to use various rudimentary techniques which are generally referred to as rules of thumb. Some will say on the low end you need 1 ton of cooling for every 500 square feet. On the high end, you will need 1 ton of cooling for every 700 square feet. These rules of thumb will also generally take into account a basic heat gain calculation for the kitchen and the number of people who will occupy the structure.

At best these rules of thumb are educated guesses and may not give you an accurate calculation of what you really need. Unless the contractor used software based on the mentioned manuals it would be to your advantage to actually do a proper load calculation for the structure based on Manual “J”, Manual “D” or Manual “N” whichever is appropriate for your HVAC application.

Air Conditioning and Heating Sizing - How many tons of air conditioning and heating do I need per square foot?

Rules of Thumb - Duct Work

Manual “D” is used for sizing residential ductwork for the appropriate amount of airflow. For each ton of air conditioning, you will need 400 cubic feet per minute (CFM) of airflow. It is imperative that this amount of airflow (400 CFM) be provided to the air conditioner evaporator coil. If the ductwork is too small it will restrict the amount of airflow crossing the evaporator coil and this will likely cause big problems. If the ductwork is too big the system may have a problem generating enough static pressure to maintain designed airflow across the coil. The proper design and construction of the air conditioning and heating ductwork are important for the proper functioning of the system. Using the proper techniques, software, and skilled personnel will ensure that the HVAC job is accomplished correctly and will provide many years of reliable and efficient comfort to you.

Air Conditioning and Heating Sizing

Using good and time-tested methods to size air conditioning and heating systems for your home or business is a smart thing to do. Using an HVAC estimator who is going to employ a rule of thumb for something that will be attached to your home and provide you with comfort for 15 to 20 years is not so smart. Use proper HVAC sizing methods and you will be better off and comfortable in the future.

Most HVAC contractors now use special HVAC software to do any load calculations. If it is a commercial job then the equipment was likely sized using software and approved by an engineer. The software takes into account all necessary rules in all the adopted manuals. The input data is collected and input into the software interface. It then generates the load calculations for sizes of the equipment, ductwork, and/or piping.

It is important, when the contractor does the calculation, that you provide them with as much information they need to make the calculation. Anything you know about your house such as insulation values will be helpful if you know. How many occupants the house typically has is also important as some software uses this in the calculation. Simply be prepared to be helpful if asked about this information.

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HVAC Random Tip Box

HVAC Tip - Did you know the human body produces approximately 250 BTU/h when sleeping? Here is a basic list of the approximate BTU/h produced by the human body for various activities:

Watching Television - 400 BTU/h (any sedentary activity)

Cleaning the house - 700 BTU/h (moderate activity)

Mowing the Grass w/push mower - 1800 BTU/h (heavy activity)

Running a marathon - 2400 BTU/h (extreme activity)

The number of people occupying a dwelling or space is a consideration in the load calculation for sizing air conditioning systems simply because humans do give off heat. BTW, the Matrix over blew the numbers. In the movie Morpheus states humans give off over 25,000 BTU's. Only the author of this website is that hot..............))))))

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